One of the most useful pieces of kit that a winemaker can have is a hydrometer. Simple devices, they are closed cylindrical glass tube weighted with steel shot at one end (steel, not mercury or lead like some sources claim). Inside the tube is a piece of paper with a scale of numbers, usually running from 0.990 to 1.100, in increments of 0.002. Because the sealed tube is hollow, it floats in liquid. Because it is weighted, the heavy end points down, ensuring that the scale is upright and readable.To use it, you carefully place it into your wine (works on beer too) and read the scale where the liquid touches the tube.
Note: while this article if from a wine blog, certainly hombrewers can find useful information too.
Many people, when asked what a hydrometer does, will answer, ‘Measures alcohol’. Some will say, ‘Measures sugar’. Neither answer is true. Hydrometers compare the the ratio of the density of the liquid to the density of water, and that’s all. It’s what we can do with this reading that’s useful to us.
If we use a standard home winemaking hydrometer on our must before fermentation, the liquid will be very high in sugar, and thus will have a density higher than that of pure water. Depending on the wine type, it could by anywhere from 1.070 to 1.110 times as dense. After we pitch yeast and the fermentation is ongoing, the sugar will be metabolised into carbon dioxide and alcohol. As the sugar levels drop, the density of the must will go down and the hydrometer won’t float quite as high. This drop shows us the progress of fermentation–which is why it’s important to record the initial gravity reading, so you can compare it. More on this below.
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